Arrays

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9 total messages Started by Stephen Boyd Wed, 18 Jun 2008 21:07

#38

Author: Stephen Boyd
Date: Wed, 18 Jun 2008 21:07

21 lines
1517 bytes

I have come across a stumper.  I have a class which contains an array. 
Like this:

   TEsCalcColorArray = array[0..7] of TColor;
   TEsCalcColors = class(TPersistent)
   public
     FCalcColors   : TEsCalcColorArray;
   end;

At some point later, I instantiate an instance of the class and try to 
access the array member.  Like this:

   FColors := TEsCalcColors.Create;
   FColors.FCalcColors[0] := clRed;

I get an exception essentially telling me that I am trying to access a 
null pointer.  If I look at FCalcColors with the debugger, it is in fact 
null.

What gives?  This is a static array.  If the containing object exists, 
then the array should exist.  Yet clearly in this case it doesn't.

Re: Arrays

#39

Author: Dennis Passmore
Date: Wed, 18 Jun 2008 21:51

17 lines
1240 bytes

How did you declare FColors ?? this code works just fine for me


type
  TEsCalcColorArray = array[0..7] of TColor;
  TEsCalcColors = class(TPersistent)
  public
    FCalcColors   : TEsCalcColorArray;
  end;

procedure TForm1.FormCreate(Sender: TObject);
var
  FColors: TEsCalcColors; // ????
begin
  FColors := TEsCalcColors.Create;
  FColors.FCalcColors[0] := clRed;
end;

Re: Arrays

#40

Author: Rudy Velthuis [TeamB]
Date: Wed, 18 Jun 2008 21:55

59 lines
3169 bytes

Stephen Boyd wrote:

> I have come across a stumper.  I have a class which contains an
> array. Like this:
> 
>   TEsCalcColorArray = array[0..7] of TColor;
>   TEsCalcColors = class(TPersistent)
>   public
>     FCalcColors   : TEsCalcColorArray;
>   end;
> 
> At some point later, I instantiate an instance of the class and try
> to access the array member.  Like this:
> 
>   FColors := TEsCalcColors.Create;
>   FColors.FCalcColors[0] := clRed;
> 
> I get an exception essentially telling me that I am trying to access
> a null pointer.

That doesn't make sense. Could it be that FColors is a field of a class
that was not instantiated yet, and that the assignment to FColors
failed?

This works:

  program Project114;

  {$APPTYPE CONSOLE}

  uses
    SysUtils, Classes, Graphics;

  type
    TEsCalcColorArray = array[0..7] of TColor;
    TEsCalcColors = class(TPersistent)
    public
      FCalcColors: TEsCalcColorArray;
    end;

  procedure Test;
  var
    FColors: TEsCalcColors;
  begin
    FColors := TEsCalcColors.Create;
    FColors.FCalcColors[0] := clRed;
    FColors.Free;
  end;

  begin
    Test;
    Readln;
  end.

-- 
Rudy Velthuis [TeamB]        http://www.teamb.com

"Why don't you write books people can read?"
 -- Nora Joyce to her husband James (1882-1941)

Re: Arrays

#41

Author: Stephen Boyd
Date: Thu, 19 Jun 2008 14:22

12 lines
1429 bytes

Rudy Velthuis [TeamB] wrote:
> 
> That doesn't make sense. Could it be that FColors is a field of a class
> that was not instantiated yet, and that the assignment to FColors
> failed?
> 

The code that I excerpted is running in the constructor of a class. 
FColors is a member of the class being instantiated.  According to the 
debugger FColors is NOT nil, as I would expect if the FColors := 
TEsColors.Create statement failed.  I would also have expected an 
exception to be thrown in that case.

Re: Arrays

#42

Author: Wayne Niddery (TeamB)
Date: Thu, 19 Jun 2008 14:41

13 lines
1869 bytes

"Stephen Boyd" <sboydlns@gmail.com> wrote in message 
news:485a6ba9$1@newsgroups.borland.com...
> The code that I excerpted is running in the constructor of a class. 
> FColors is a member of the class being instantiated.  According to the 
> debugger FColors is NOT nil, as I would expect if the FColors := 
> TEsColors.Create statement failed.  I would also have expected an 
> exception to be thrown in that case.

Let's see the code that is instantiating the containing class.

-- 
Wayne Niddery - TeamB (www.teamb.com)
Winwright, Inc. (www.winwright.ca)

Re: Arrays

#43

Author: Marc Rohloff [TeamB]
Date: Thu, 19 Jun 2008 15:12

9 lines
1444 bytes

On Wed, 18 Jun 2008 17:51:35 -0400, Dennis Passmore wrote:

> How did you declare FColors ?? this code works just fine for me

This doesn't work for me. Did you test this under .NET?

-- 
Marc Rohloff [TeamB]
marc -at- marc rohloff -dot- com

Re: Arrays

#44

Author: Marc Rohloff [TeamB]
Date: Thu, 19 Jun 2008 15:25

14 lines
1524 bytes

On Wed, 18 Jun 2008 17:07:41 -0400, Stephen Boyd wrote:

>    TEsCalcColorArray = array[0..7] of TColor;
>    TEsCalcColors = class(TPersistent)
>    public
>      FCalcColors   : TEsCalcColorArray;
>    end;

You need to create an 'empty' constructor that only calls inherited.
http://qc.codegear.com/wc/qcmain.aspx?d=25580

-- 
Marc Rohloff [TeamB]
marc -at- marc rohloff -dot- com

Re: Arrays

#45

Author: Stephen Boyd
Date: Thu, 19 Jun 2008 16:05

17 lines
1629 bytes

Marc Rohloff [TeamB] wrote:
> On Wed, 18 Jun 2008 17:07:41 -0400, Stephen Boyd wrote:
> 
>>    TEsCalcColorArray = array[0..7] of TColor;
>>    TEsCalcColors = class(TPersistent)
>>    public
>>      FCalcColors   : TEsCalcColorArray;
>>    end;
> 
> You need to create an 'empty' constructor that only calls inherited.
> http://qc.codegear.com/wc/qcmain.aspx?d=25580
> 
That fixed it.  Thank You.  That was driving me nuts!  I thought that 30 
years of programming had finally destroyed what little mind I have left. ;)

You would think that the good folks at CodeGear would have fixed that by 
now and issued an update.

Re: Arrays

#46

Author: Rudy Velthuis [TeamB]
Date: Thu, 19 Jun 2008 16:58

14 lines
2312 bytes

Marc Rohloff [TeamB] wrote:

> You need to create an 'empty' constructor that only calls inherited.
> http://qc.codegear.com/wc/qcmain.aspx?d=25580

Aargh! I had not noticed this was .dotnet. Indeed, you are right.

This issue does not arise in Win32.
-- 
Rudy Velthuis [TeamB]        http://www.teamb.com

"Every day I get up and look through the Forbes list of the 
 richest people in America. If I'm not there, I go to work."
 -- Robert Orben

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